The problem with writing about sea slug colouration is that they are so spectacular, so lovely, that you are tempted to turn the whole post into a photo gallery just to show off the pictures.
The bright colours are not there for just the benefit of lucky SCUBA divers. As I’ve noted before, sea slugs mollusks are perhaps the most appealing targets imaginable for a predator. When your entire body plan is essentially a predator’s “meals on wheels,” you might think the last thing you would want to do would be to draw attention to yourself.
Except that seas slugs are not as helpless as they might seem. They use chemicals extracted from the food they eat to make fairly nasty new chemicals. Predators do not like ingesting these chemicals, and some may well be toxic enough that if you ate one of these slugs, the predator might die.
Of course, making a predator sick after you’re in said predator’s digestive gland doesn’t do the slug any good, either. And that’s the problem with deterrents: they only work when everyone knows about them. (Corollary: If you ever develop a doomsday device, don’t keep it a secret.)
The theory that these bright colours are honest signals is more often intuited that tested. Just because SCUBA divers find sea slugs easy to see doesn’t mean that other animals do. Every animal has different sensory abilities, a different umwelt than humans. For instance, some fish see ultraviolet; many crustaceans are insensitive to red. And just because they seem very bright in an aquarium doesn’t mean they’ll stand out in the wild, where shades of the rainbow drop out the deeper you go.
You also have to reckon that at some point, there is going to be a point of diminishing returns. Even if you have a deterrent, there’s a point where you will draw so much attention to yourself that predators will attack either because they’re naive or starving or in error. (Corollary: Don’t go around being an ass even if you know kung fu, because sooner or later someone will wail on you anyway for being a jerk.)
Cortesi and Cheney keep quite a few of these balls in their air in this paper to figure out if, in fact, sea slugs’s bright colours are warning signs as predicted. They collected twenty sea slug species from their natural habitats, and took high quality images, including the ultraviolet, of both the slug and the environment it was found in.
To test whether this combination of colours of the slug on the background were conspicuous to fish, they used models of the kinds of colours that a couple of different fish species can see. The triggerfish (here) sees in the blue-green range; another species tested sees up into the ultraviolet. The authors admit that these species were chosen because we know about their visual systems, not because we know for certain that they eat all twenty of these sea slug species. Somewhere down the road, it would be nice if someone did behavioural tests with known sea slug predators.
The theory is that the most conspicuous sea slugs should be the ones that are the most toxic. To test toxicity, the authors ground up slugs, and put the extract in with brine shrimp eggs. Brine shrimp are pretty tough little creatures, but development is fairly fussy, and shrimp will be killed off by sea slug chemicals.
One of the most toxic is Sagaminopteron ornatum, shown here. Hypselodoris obscura, below that, is near the bottom of the toxicity list; the very bottom one I couldn't find a picture of searching the photo database as I usually do. I guess it’s just that plain-looking that nobody can be bothered to take a picture of it.
There is a correlation between conspicuousness and toxicity, regardless of what fish visual system they used to model this. There is a lot of variation, though, with the correlation explaining only about 7-12% of the variance.
The authors talk about several explanations to the rest of the variance. They talk about sexual selection, developmental variation, and the necessary simplification of the rampantly wild colour patterns into a more tractable measure of contrast. They also point out that while they measured toxicity, toxicity may not be as relevant a measure as palatability. There are plenty of foods that wouldn’t kill me or even make me sick, but I sure wouldn’t choose to eat voluntarily.
One of the other factors that they don’t raise is that there may be mimicry going on. Typically mimicry is considered to be between a pair of, or small number of species. But maybe there’s an element of bluffing that could be effective.
The authors also don’t discuss the possibility of individual variation. The methods are a bit vague on just how many individuals they used to test the toxicity. Considering that the chemical defenses are diet based, it’s entirely possible that toxicity fluctuates not only between individuals, but within an individual over time, as well.
Additional: Forgotten I’d bookmarked this lovely gallery from National Geographic. Like I said, you can’t help but end up making a slideshow.
Cortesi, F., & Cheney, K. (2010). Conspicuousness is correlated with toxicity in marine opisthobranchs Journal of Evolutionary Biology DOI: 10.1111/j.1420-9101.2010.02018.x
Chromodora splendida picture by taso.viglas on Flickr.
Chelidonura varians by Allerina & Glen MacLarty on Flickr.
Rhinecanthus aculeatus picture by zsispeo on Flickr.
Sagaminopteron ornatum by saspotato on Flickr.
Hypselodoris obscura by doug.deep on Flickr.
All photos used under a Creative Commons license.
Video link I want to embed, but can't.